System and method for determining depth of chest compressions

ABSTRACT

Systems and methods for determining depth of compressions of a chest of a patient receiving chest compressions. A field detector is used having at least two coils at a fixed distance from each other.

CROSS-REFERENCE TO RELATED APPLICATIONS

This disclosure claims priority from U.S. Provisional Application No.61/158,002, filed Mar. 6, 2009, and U.S. Provisional Application No.61/235,584, filed Aug. 20, 2009, the disclosures of which are herebyincorporated by reference in its entirety.

TECHNICAL FIELD

This disclosure relates to systems and methods for determining depth ofchest compressions, for example during the administration ofcardiopulmonary resuscitation (CPR). In particular, this disclosurerelates to determination of chest compression depth by use of a positionsensor and a reference sensor.

BACKGROUND

There are currently an estimated 40,000 incidences of cardiac arrestevery year in Canada, most of which take place outside of hospitalsettings. The odds of an out-of-hospital cardiac arrest currently standat approximately 5%. In the U.S., there are about 164,600 such instanceseach year, or about 0.55 per 1000 population. It may be desirable todecrease the number of deaths resulting from these out-of-hospitalincidences of cardiac arrest. Certain places, such as sports arenas, andcertain individuals, such as the elderly, are at particular risk and inthese places and for these people, a convenient solution may be thedifference between survival and death.

Cardiopulmonary resuscitation (CPR) is a proven effective technique formedical and non-medical professionals to improve the chance of survivalfor patients experiencing cardiac failure. CPR forces blood through thecirculatory system until professional medical help arrives, therebymaintaining oxygen distribution throughout the patient's body. However,the quality of CPR is often poor. Retention of proper CPR technique andprotocol may be inadequate in most individuals and the anxiety of anemergency situation may confuse and hinder an individual in deliveringproper treatment.

According to the Journal of the American Medical Association (2005),cardiopulmonary resuscitation (CPR) is often performed inconsistentlyand inefficiently, resulting in preventable deaths. Months after thecompletion of standard CPR training and testing, an individual'scompetency at performing effective chest compressions often deterioratessignificantly. This finding was found to hold true for untrainedperformers as well as trained professionals such as paramedics, nurses,and even physicians.

The International Liaison Committee on Resuscitation in 2005 describedan effective method of administering CPR and the parameters associatedwith an effective technique. Parameters include chest compression rateand chest compression depth. Chest compression rate is defined as thenumber of compressions delivered per minute. Chest compression depth isdefined as how far the patient's sternum is displaced by eachcompression. An effective compression rate may be 100 chest compressionsper minute at a compression depth of about 45 cm. According to a 2005study of actual CPR administration at Ulleval University Hospital inNorway, on average, compression rates were less than 90 compressions perminute and compression depth was too shallow for 37% of compressions.

Therefore, a system to facilitate the proper delivery of CPR in anemergency may be useful. Furthermore, a system that can also be used inobjectively training and testing an individual may be useful for the CPRtraining process and protocol retention.

Most existing CPR assist technologies use accelerometers for thedetermination of compression depth. One such device is disclosed in U.S.Pat. No. 7,074,199. However, any acceleration data from accelerometersused to measure the depth of chest compression during CPR is prone tocumulative errors and drift errors. Consequently, these sensors are notsuitable for highly accurate or detailed data collection regarding CPRparameters and can only be relied on for approximate depth values.Furthermore, the use of an accelerometer in a CPR monitoring devicewithout an external reference is prone to error if the patient or CPRadministrator is mobile. For example, if the patient is being medicallytransported in an ambulance, helicopter or on a gurney, theaccelerometer is unable to differentiate between the external movementof the patient and the compressions of the chest. In any type ofnon-stationary environment, an accelerometer based device may beunreliable and ineffective. The use of an accelerometer to calculatecompression depth also relies on complicated and error-pronecalculations to compensate for the angle and tilt of the compressiondevice. If the accelerometer is not perfectly level on the chest of thepatient and its movement is not perfectly vertical, errors mayaccumulate and must be accounted for by the angle of the two horizontalaxes. Furthermore, the absence of any external reference point makes itdifficult for the device to know its position in space at any giventime. All measurements of distance are relative and an origin ofmovement is difficult to ascertain and maintain over the course ofmeasurements. This may cause the initiation or starting point of thecompressions to drift over time leading to errors in depth measurements.Certain commercial products currently use accelerometer technology, suchas the AED Plus® D-Padz® from Zoll Medical, in which the accelerometeris embedded into the pads of the defibrillator. Due to the additionalcircuitry and sensory within them, these defibrillator pads aresubstantially more expensive and must be disposed of after each use.Therefore, relatively expensive sensory must be routinely discarded dueto the design of the product.

U.S. Patent Application Publication No. 2007/0276300 to Kenneth F. Olsonet al. discloses a device using ultrasound transmission to calculatecompression depth. An acoustic signal is transmitted from a device onthe chest of the patient to a receiver in another location. This devicehas several drawbacks. First, the ultrasound signal must have a clearline of sight from transmitter to receiver in order to operate. Anyinterference, objects, people or even the hand of the user in the way ofthe signal may result in signal loss or deterioration. The transmittermust be directed toward the receiver and the relative orientationbetween the transmitter and receiver is crucial. Second, ultrasound isrelatively slow and a time-of-flight measurement of an ultrasound signalmay suffer from significant lag and latency. Third, an ultrasound signalis highly dependent on ambient conditions such as air temperature. Ifair temperature fluctuates, so does the speed of sound, which may resultin inaccuracies. Finally, if the plane of the chest compression isinitially unknown, the calculation of compression depth may besignificantly compromised. Time-of-flight ultrasonic distanceinterpolation cannot resolve the position of the receiver in six degreesof freedom and the determination of the downward translational movementif the patient, receiver or transmitter is not level may be difficult.Even if ultrasonic triangulation is employed, latency may besignificant, resolution may be low and multiple transmitters andreceivers in different locations may be required.

Existing CPR assist devices and systems are relatively ineffective atmeasuring chest recoil. Chest recoil is the extent to which the chest isreleased following a compression. For a chest compression to becompletely effective, the chest must be fully released before beginninganother compression. When a compression is released, elastic recoil willcreate a negative pressure that pulls blood into the chest. Incompletedecompression will reduce the amount of blood available to be circulatedwith the next compression. Accelerometer-based devices lack the abilityto establish a reference point at the top of a compression that may beused to adequately measure recoil. As there is no external reference,the accelerometer signal may drift over time and the device may becomeineffective at determining whether the chest has been fully released.

A recent study (Resuscitation. 2009 January; 80(1):79-82. Epub 2008 Oct.25: ‘Compression feedback devices over estimate chest compression depthwhen performed on a bed’) has unearthed another inadequacy in currentCPR assist devices. The study indicates that CPR assist devices tend tooverestimate compression depth when the patient is on a mattress. Thedevice tends to erroneously register the movement of the mattress aspart of the chest compression.

Other CPR assist tools use mechanical force measurements as anindication of compression depth. These devices may be inaccurate due totheir inability to compensate for varying chest compliances. They tendto rely on the user's subjective impression of the patient's body sizeto help calibrate the proper amount of force to be administered.Furthermore, a recent study (Resuscitation. 2008 July; 78(1):66-70. Epub2008 Apr. 18: ‘Does use of the CPREzy involve more work than CPR withoutfeedback?’) has shown that these devices tend to require more work thanCPR without an assist tool due to the device's internal mechanism. Thespring within the device may add an additional 20% workload to the CPRprocess leading to a faster onset of user fatigue.

Presently available CPR assist devices and system typically suffer froma major disadvantage. They tend to indirectly measure depth by firstdetermining acceleration, velocity or force. Ultimately compressiondepth is a measure of position and the determination of accelerationrequires doubly integrating the received signal to obtain useful data.Such integration introduces a significant source of error into themeasurement. It may be desirable to provide a method of determining CPRcompression depth by measuring position, rather than acceleration,velocity or force. By measuring position directly, errors related tointegration of the signal or compliance of the patient's chest are notintroduced. The position data may be used to directly calculate thedepth of chest compressions.

It may be desirable to provide an easy-to-use and inexpensive system toaccurately measure relevant CPR parameters such as compression depth andrate absent of the problems in the aforementioned technologies.

SUMMARY

The present disclosure is directed to a method and system fordetermination of compression parameters during administration of CPR.The system includes the features and methods disclosed in patentapplications Nos. 61/158,002 and 61/235,584, and provides additionalprocessing strategies and hardware components. The aforementionedapplications describe the use of a field generator and a field detector.The generator and detector may be used as a reference sensor and aposition sensor. The reference sensor is relatively stationary, whilethe position sensor is placed on the patient's chest and moves accordingto each chest compression. The field generator and field detectorspecifically generates and detects a field, such as an electromagneticfield, rather than simply transmitting and receiving a signal.

In some aspects, there is provided a compound field detector fordetermining a depth of compression of a chest of a patient receivingchest compressions, the detector comprising: at least two coils at afixed distance from each other; wherein the detector is adapted togenerate a response signal indicative of any one of the at least twocoils detecting a field.

In some aspects, there is provided a method for determining a depth ofcompression of a chest of a patient receiving chest compressions, themethod comprising: determining the positions of at least two coilsadapted to move in accordance with the chest; estimating an apparentdistance between the coils; estimating a correction factor based on anydifferences between the apparent distance and a known distance betweenthe coils; and determining the chest compression depth based on thedetermined positions and the correction factor.

In some aspects, there is provided a system for determining a depth ofcompression of a chest of a patient receiving chest compressions, thesystem comprising: a field generator adapted to generate a field; acompound field detector including at least two coils at a fixed distancefrom each other, the field detector adapted to generate a responsesignal indicative of any one of the at least two coils detecting thefield; and a processor adapted to determine from the response signalposition information for the field detector relative to the fieldgenerator, and to determine the chest compression depth from thedetermined position information; wherein one of the field generator andthe field detector is a position sensor adapted to move in accordancethe chest as the chest is receiving the compressions, and the other ofthe field generator and the field detector is a reference sensor adaptedto be stationary relative to the patient.

Due to the relatively uniform and predictable nature of chestcompressions, various computational algorithms may be used to relativelyaccurately calculate CPR parameters. Expected sources of error such assignal jitter and distortion caused by highly ferrous and conductivemetals may be reduced or eliminated through the implementation ofequations and/or filtering techniques tailored to the properties ofchest compressions.

Beyond computational algorithms, other techniques may be employed toreduce or eliminate error, noise and/or distortion in the measured CPRparameters. As in software, the unique properties associated with themovement of a chest compression may enable unique hardware designs thatresult in cleaner, more reliable position and/or depth estimates.Specifically, the field generator and field detector coils may beconfigured to reduce the effects of metallic disturbances in theoperational environment. In some examples, a compound detector may beused in which two or more detectors in a fixed relationship are usedtogether. The known separation distance between the centers of these twoor more detectors may be used to detect and compensate for distortion inthe environment (e.g., due to metallic objects). In some examples, theaddition of a second sensing modality may assist in further reducingerror by detecting the presence of distorted or otherwise incorrectdata. For example, a pressure or force sensor may detect errorsoriginating from sources that do not affect the pressure or force sensorbut do affect the field coils (e.g., distortion from metallic objects).

In some aspects, there are provided methods that may be employed tocompensate or correct for potential sources of system error or failureresulting from external perturbations of the system. For example, if thereference sensor is moved during the administration of CPR, the systemmay detect such a movement and recalibrate the depth calculationaccordingly to compensate for the movement. In some examples, the systemmay continue to operate during this disturbance: The continuousoperation during movement of the reference sensor may be accomplishedthrough the addition of an external sensor unaffected by the movement,such as a force or pressure sensor.

In some aspects, there are also provided methods for the compensation ofunsatisfactory or error-prone environments in which the system may beoperated. For example, the system may be configured to compensate for asituation in which the patient is supported on a non-rigid surface, suchas a mattress. A non-rigid surface may exhibit motion or displacementduring administration of CPR, which may result in erroneous measurementsif not taken into account. For example, during CPR, the mattress belowthe patient is compressed along with the chest. This may result in anerroneously large compression depth measurement that is not indicativeof the actual depth that only the chest, in the absence of the mattress,is compressed. The use of an additional field detector may overcome thisproblem. Given the three components of a first detector, a seconddetector and a generator, one component may be adapted to move inaccordance with the non-rigid surface (e.g., placed on the mattressbelow the patient), a second component may be adapted to move inaccordance with the patient's chest (e.g., placed on the patient'schest), and the third component may be adapted to be stationary relativeto the patient. The actual depth of compressions may then be determinedby determining the relative motion between the component (e.g., thefirst detector) moving with the non-rigid surface and the component(e.g., the second detector) moving with the patient's chest, for exampleby subtracting the position of one from the position of the other.

In some examples, depending on the specific situation, the system may beadaptable to the emergency. For example, the position sensor may beremovably housed in a sheath or housing. The sheath may protect theuser's hands while providing additional comfort. In some examples, thesheath may be removed from the position sensor when the system is usedto perform CPR on an infant. The removal of the sheath may transform theposition sensor from an adult-sized pad to an infant-sized pad. In someexamples, the position sensor may also be affixed or housed in a numberof items found at an emergency scene. For example, one or both of thegenerator and detector may be affixed to a patient backboard, embeddedwithin the electrodes of a defibrillator or attached to a gurney or ahospital bed.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure will be discussed in detail below,with reference to the drawings in which:

FIG. 1 is an illustration of a CPR assist system in accordance with anembodiment of the present disclosure;

FIG. 2 is a top plan view showing a field detector of a CPR assistsystem within a pad on the patient's chest;

FIG. 3 is an illustration of a field generated by an example fieldgenerator and detectable by an example field detector suitable for anexample embodiment of the CPR assist system;

FIG. 4 is a flowchart showing an example algorithm for the calculationof position;

FIG. 5 is an illustration showing an example method of calculating chestcompression depth by first forming a plane in space;

FIG. 6 is a diagram showing a normal vector parallel to the path of achest compression and its corresponding plane in space;

FIG. 7 is an illustration showing an example pad housing an exampleposition sensor and the plane bisecting the pad;

FIG. 8 is an illustration of an example pad housing an example positionsensor in which the coordinate axes are labeled on the pad;

FIG. 9 is an illustration showing an example embodiment of the CPRassist system in which a second field detector is placed in a stationaryposition in the environment;

FIG. 10 is an illustration showing two example position sensors placedin fixed relation to each other, in a compound detector;

FIG. 11 is a diagram showing the normal vector joining the centres ofthe position sensors in the configuration of FIG. 10;

FIG. 12. is an illustration showing the movement of each field detectorin the configuration of FIG. 10;

FIG. 13 is an illustration showing two example detectors placed inanother example compound detector;

FIG. 14 is an illustration of another example compound detector in whicheach of the detectors shares one or more common coils;

FIG. 15 is an illustration of an example compound detector in which thedetectors do not share any common coils;

FIG. 16 is an illustration showing a compound detector moving frombetween two quadrants in space;

FIG. 17 is an exploded view of an example compound detector having anexample force sensor;

FIG. 18 is an illustration showing an example base unit containing anexample reference sensor placed in a holder on the wall of an ambulanceand an example position sensor on the chest of a patient;

FIG. 19 is an illustration of an example base unit with an exampledetachable position sensor in its off configuration;

FIG. 20 is an illustration of an example base unit with an exampledetachable position sensor detached from the unit and ready for use;

FIG. 21 is an illustration showing an example field generator detachedfrom its base unit and placed under the patient and an example fielddetector placed on the patient's chest;

FIG. 22 is an illustration showing an example field generator in a baseunit and two example field detectors, one placed under the patient andone on the patient's chest;

FIG. 23 is an illustration showing an example reference sensor placedunder the patient and an example position sensor placed on the patient'schest with feedback provided on an example position sensor;

FIG. 24 is an illustration showing an example patient backboard with anexample reference sensor embedded in the backboard;

FIG. 25 is a diagram showing the angles of rotation of an example padhousing an example position sensor;

FIG. 26 is an illustration of a user performing CPR in an example ofproper form;

FIG. 27 is an illustration showing an example wearable embodiment of theCPR assist system;

FIG. 28 is an illustration showing an example reference sensor positionon or adjacent to a pulse oximetry device;

FIG. 29 is an illustration showing a chest impedance measurement betweenthe pads of a defibrillator with an example position sensor embedded inone of the pads;

FIG. 30 is an illustration showing an example removable housing for anexample position sensor;

FIG. 31 is an illustration showing an example position sensor withfeedback provided on the position sensor; and

FIG. 32 is an illustration showing an example base unit displaying anexample feedback prompt.

DETAILED DESCRIPTION

The present disclosure is directed to a method and system for thedetermination and calculation of chest compression parameters, such aschest compression depth, during the administration of cardiopulmonaryresuscitation (CPR). The system may also be referred to as a CPR assistsystem.

The system includes a field generator and a field detector. In someembodiments, the field detector is a position sensor and the fieldgenerator is a reference sensor. The position sensor may be placed at alocation that corresponds to movement of the patient's chest, while thereference sensor may be placed at a relatively stationary location.Signals, for example electromagnetic fields, are generated by thereference sensor and detected by the position sensor. In otherembodiments, the field detector is the reference sensor and the fieldgenerator is the position sensor, in which case signals, which may befields, are generated by the position sensor and detected by thereference sensor. It would be clear to a skilled person that theposition sensor and reference sensor are interchangeable. A processor inthe system determines the position of the position sensor relative tothe reference sensor based on the signal. Based on the determinedposition, the processor determines the chest compression parameters,including chest compression depth, during administration of CPR.

Reference is now made to FIGS. 1 and 2. In this example, the CPR assistsystem may include a relatively stationary base unit 1 containing areference sensor 4 in the environment of an emergency and a positionsensor 2 that may move according to a patient's chest movement, relativeto the reference sensor 4, thus tracking the movement of the chest ofthe patient 3 during CPR. In this example, the reference sensor 4 is thefield generator and the position sensor 2 is the field detector. Thereference sensor 4 is capable of generating a signal, such as a field 5,that is detected by the position sensor 2. In this example, the positionsensor 2 is provided in a structure placed on the chest of the patient,such as a block, pad 6 or other suitable structure and is connected tothe base unit 1 by a cable 8. The CPR administrator or user 9 maycompress the chest of the patient directly by placing his or her hands 7on the pad 6. Here, the base unit 1 is placed on the ground 10, which isrelatively stationary relative to the patient. Although the base unit 1is shown, in some examples the system does not include a base unit.

As shown in FIG. 3, the field detector (e.g., the position sensor in theabove example) is configured to sense the generated field from the fieldgenerator (e.g., the reference sensor in the above example). The fielddetector may then produce a response signal. A processor determines theposition of the position sensor, for example its three-dimensionalposition coordinates, relative to the reference sensor, based on theresponse signal. The processor may be provided together with thereference sensor in the base unit, may be provided on the positionsensor, or may be a separate component. The processor may receiveinformation from the position sensor through wired or wirelesscommunication. For example, the position sensor may include a wire forcoupling with the reference sensor and/or the processor, or may includea wireless transmission component for wireless coupling with thereference sensor and/or the process. Similarly, the reference sensor maycommunicate with the processor through wired or wireless communication.

The determination of the position sensor's coordinates may beaccomplished by measuring the strength of the field detected from thefield generator. The position information may be used by the processorto determine a chest compression parameter, such as chest compressiondepth. The chest compression parameter (e.g., chest compression depth)may be provided to the user through a feedback component, for examplethrough audible, visual and/or tactile feedback. Chest compression depthmay be determined from the position information by determining initialand final positions corresponding to the start and end of a singlecompression and subtracting one from the other. Other commoncalculations may be used for determination of chest compression depth.

Possible hardware and software that may be used to calculate threedegree of freedom coordinate position information is disclosed in U.S.patent application Ser. No. 12/354,913, the disclosure of which ishereby incorporated in its entirety by reference. That applicationdiscloses methods of demodulating and filtering the response signal togenerate a 3×3 signal matrix representative of the ninegenerator-detector couplings. Calculating position may be accomplishedusing methods similar to those disclosed in U.S. Pat. No. 4,314,251, thedisclosure of which is hereby incorporated in its entirety by reference.One method of calculating position uses a three-axis field detector. Thethree-axis sensor determines the complete signal vector produced at theposition sensor location by each excitation vector of the fieldgenerator. Orientation of the position sensor relative to the referencesensor is initially unknown and position may, therefore, be determinedfrom signal parameters that are unaffected by sensor orientationunknowns. Solutions for the unknown position of the sensor may beformulated in terms of squared magnitudes and the dot products of sensoroutput vectors. Both of these quantities are invariant under sensorrotation.

For example, the magnitudes of the three coordinates of the position ofthe sensor may be determined through a system of equations based uponthe outputs of a three-axis position sensor produced by all threeexcitation vectors. Trigonometric relationships and position-framesensor-output vectors corresponding to the excitation vectors may beused to determine relationships between the squared vector magnitudesand the x, y and z coordinates.

Once the x, y, and z values are determined, the coordinates may bedenormalized if appropriate. The signs of the x, y and z coordinates aredetermined by the dot products of the sensor output vectors. The processfor calculating x, y and z may be modified for sensors and sources offewer or greater than three axes. One example method of calculating theposition of the position sensor is shown in FIG. 4.

Upon obtaining x, y and z coordinates for the position of the sensor,the calculation of compression depth and other parameters may beperformed. A number of methods may be used to calculate chestcompression depth. For example, where the field generator provides thereference frame for the system, the calculation of chest compressiondepth may be accomplished by forming an imaginary plane 11 in space thatis substantially parallel to the patient's chest as shown in FIG. 5.This plane 11 forms a reference location for the start of a chestcompression and the chest compression depth is calculated as a distancefrom the plane. The equation of the plane may be calculated by formingan initial vector 12 along the downward motion of the first compression.A starting, reference or “home” coordinate 13 and one or more othercoordinates 14 along the length of the compression may determined andstored in memory as shown in FIG. 6. The normal vector 12 for the planemay be calculated using these coordinates. The equation of a plane isAx+By+Cz+D=0 where (A,B,C) is the vector normal to the plane. The valueof D may be calculated by substituting the “home” coordinates into theplane equation and solving. The calculated plane represents the highestdepth level that the position sensor may achieve during a chestcompression. For example, the plane may be substantially parallel to thepatient's chest and substantially parallel to the top surface of the pad6 housing the position sensor 2 as shown in FIG. 7. The compressiondepth may then be calculated as the current coordinate's distance fromthat plane. The distance to the plane from any current coordinates (x,y, z) is calculated as:

$d = \frac{{Ax} + {By} + {Cz} + D}{\sqrt{A^{2} + B^{2} + C^{2}}}$

The sign of d may be used to determine if the current compressioncoordinate is above or below the starting position of the chestcompression. Any value indicating a compression position above the planeis likely to be erroneous or may represent movement of the pad that isnot part of the chest compression. Adequate chest recoil may also becalculated by ensuring the user sufficiently releases the chest of thepatient such that the sensor returns to the starting plane position.

Another example method of determining the depth of a chest compressionis to use the position sensor to provide the reference frame forcalculations. If the position sensor is the reference frame, rotationsof the reference sensor will not affect the x, y, z coordinates of theposition sensor. Therefore, the initial rotation of the reference sensormay not be relevant to the current x, y, z position of the positionsensor as the position sensor is moving in its own reference frame. Thecalculations may be moved from the reference sensor frame-of-referenceto the position sensor frame-of-reference by simple rotations known tothose skilled in the art. The starting configuration (i.e. orientation)of the position sensor may be known relative to the chest of thepatient. For example, in a CPR assist system, the pad 6 housing theposition sensor may be marked with lines 15 indicating where on thepatient's chest the pad should be placed as shown in FIG. 8. The pad maybe placed, for example, between the nipples of the patient and on thesternum. Since the position sensor frame-of-reference is the referenceframe, all movements of the position sensor are relative to its owncurrent position and configuration. Therefore, the coordinate positionscalculated during a chest compression may be used to directly determinecompression depth. The actual x, y, z trajectory of the position sensorthrough three dimensional space may be tracked as the compression isadministered, or the distance between the starting coordinate of thecompression and each current coordinate may be calculated to determinethe present compression depth.

By tracking the coordinates through space, rather than calculating thedistance between points directly, other algorithms may be developed thatare able to account for lateral shifting of the system and/or othermovements that are not part of the chest compression. For example, asideways movement of the position sensor 2 may be misconstrued as aportion of the vertical movement of the compression. Errors resultingfrom these spurious movements may be reduced or eliminated by monitoringthe three dimensional trajectory of the position sensor as it movesthrough the chest compression. Lateral, non-compression components ofmotion may be eliminated from the calculated depth. This may be anadvantage of measuring the x, y, z position of the position sensordirectly over traditional systems employing accelerometers and forcesensors that cannot easily differentiate such spurious movements.

In another example method of calculating compression depth, the fieldgenerator is adapted to move in accordance with the patient's chest(e.g., in a pad placed on the chest) and the field detector is adaptedto be stationary relative to the patient. Since the field generator isthe reference frame for the system, all position data will be in theframe of the pad on the chest of the patient. Therefore, as above, thepad may be marked with landmarks or reference points to align the padproperly on the chest. The known configuration of the pad relative tothe chest may allow the present position and depth of the chestcompression to be relatively accurately monitored. Unlike in theprevious method, this technique does not require rotating thecoordinates into the position sensor frame as the field generator is theposition sensor and all measurements are relative to the fieldgenerator.

As described previously, either the field detector or the fieldgenerator may be used as the position sensor, and the other of the pairmay be used as the reference sensor. Multiple reference and/or positionsensors may be used, which may further improve the accuracy of theposition and orientation information. For example, in an environmentwith a significant source of noise or interference (e.g., in the casewhere the system uses electromagnetic fields as signals, a noisyenvironment may be one containing significant sources of metal), asecond field detector 16 may be placed in the environment of the firstfield detector 17, where the first field detector is used as a positionsensor and the reference sensor is a field generator as shown in FIG. 9.This second field detector may be relatively fixed in position and usedto calibrate the measurements of the system by determining the ambientinterference in the environment. For example, within an ambulance, anexisting field detector may be fixed within the environment and may beable to measure the ambient distortion present within that environment.

Electromagnetic tracking systems, such as the one described in U.S. Pat.No. 4,313,251, the disclosure of which is hereby incorporated in itsentirety by reference, may suffer from a major limitation. Theelectromagnetic signals generated by these systems are typically proneto distortion caused by the presence of metallic objects. The majorsources of distortion are primarily large, conductive metallic objects.There are two properties of a metal that determine the extent to whichit will distort an electromagnetic field. The first property is theconductivity of the metal. Varying fields, such as sinusoidalelectromagnetic fields, generate eddy currents in conductive materials.The extent to which eddy currents are produced is dependent on the sizeand conductivity of the material. Very conductive metals, such ascopper, are more threatening to the field than less conductive metalssuch as stainless steel. The second property is the permeability of themetal. Materials that are highly permeable at the frequency of thegenerated field may skew the detected field.

Cautionary steps may be taken with current electromagnetic trackingsystems to reduce metallic interference. For example, the distancebetween the field generator or field detector and any large metallicobject may be increased until the effect is negligible. Alternatively,the separation distance between the field generator and field detectormay be minimized thereby reducing the distortion caused by any nearbymetal. It is also possible to map out all the sources of metal in theenvironment prior to collecting data. However, these and other existingmethods of distortion compensation may not be practical in a number ofoperating situations. For example, in a real-world environment, it isoften difficult and cumbersome to ensure all sources of metal arecompletely removed. It is also often difficult and time consuming tolocate and map each and every metallic source prior to operation of thesystem. In applications where metal may be present, but fast andreliable operation is required, mapping is often not a practical option.

Certain tracking applications require only relative measurements ofposition and involve a relatively predictable trajectory of motion. Forexample, the application may require tracking over a simple, linear pathsuch as the path traveled by the chest of a patient during theadministration of CPR. During CPR, the vector along which the chest willtravel is substantially known (i.e. the chest will be compressed towardthe spine along a substantially straight, downward path). The propertiesof this linear motion along a substantially known vector may be used toimprove the accuracy of the data while reducing or eliminatingdistortion from metallic noise and/or interference from other electricaldevices.

For example, reducing the effects of metallic distortion while improvingaccuracy in an electromagnetic system where the object to be trackedmoves along a known vector path (such as in CPR), may be accomplishedthrough the use of a compound detector having at least two coils in afixed distance from each other. The compound detector behaves similarlyto a simple (i.e., non-compound) detector, however response signals fromthe compound detector may be generated by either one or both of thecoils. For example, a second field detector 16 may be affixed directlybelow the first field detector 17, both moving together as one unit,also referred to as a “stacked detector” or compound detector 18, asshown in FIG. 10. In the example compound detector 18, there are atleast two coil assemblies (in this case, field detectors 16, 17) havingwindings substantially parallel to each other and spaced apart from eachother at a fixed and known distance. Although the compound detector 18is described as having two or more detectors, it should be understoodthat the compound detector 18 may have two or more spaced apart coils orcoil assemblies rather than detectors. The known spacing is in adirection substantially parallel to the direction of expected motion (inthis case, perpendicular to the parallel windings). In the exampleshown, the two detectors 16, 17 may be placed directly on top of eachother so that corresponding coils are parallel to each other and so thatthe centre of one detector 16 is directly above the center of the otherdetector 17. A longitudinal axis 19 that is perpendicular to the planesdefined by each of the windings and connecting the centres of each fielddetector 16, 17 may run substantially directly along or parallel to theexpected path of the motion (i.e. path of chest compression) to bemeasured as shown in FIG. 11. Because the two detectors 16, 17 arestacked and fixed together, the distance between the centres of the twofield detectors 16, 17 is a known and fixed constant value.

The raw data from each of the two field detectors in the compounddetector 18 may be correlated to obtain more accurate positioninformation. The known separation distance may be used to detect sourcesof distortion and/or noise in the environment, and used to correct forthe distortion and/or other sources of noise. Although two fielddetectors are shown in the compound detector, more than two fielddetectors may be used, provided the distances among the field detectorsare all known and fixed. Where there are more than two detectors in thecompound detector, the field detectors may all be distanced from eachother along the same direction (e.g., parallel to the expected directionof compression) or along different directions. Where the field detectorsare distanced from each other in different directions, such aconfiguration may be useful for determining and correcting fordistortions in multiple directions.

There are a number of ways in which the coils within the compounddetector may be used to compensate for metallic objects present in theenvironment. For example, when a conductive or ferrous metal is close tothe detector or generator, the measured absolute position from thesource to each of the field detectors will be distorted. The distortionwill cause the known and fixed distance between the two field detectorsbe detected as being apparently closer lesser or greater, depending onthe type of distortion present. This discrepancy between the knownactual fixed distance and the measured apparent distance is an indicatorof the type and magnitude of distortion present along the path ofmotion. This information may be used to calculate the effect of thedistortion on the detector as it moves through a motion along the vectorjoining the centers of the detector coils. As the vector joining the twodetector's centres is substantially aligned or parallel with the axis ofthe motion, the distortion along that vector and hence, along the lengthof the motion may be determined.

For example, if the actual distance between the detector coils in thecompound detector configuration is five millimetres and the measuredapparent distance is ten millimetres, a scaling factor of two may beused on the motion measured over the distance separating the coil.Therefore, if the detector has a measured apparent movement of fourmillimetres, its actual movement may be corrected to be actually twomillimetres. Although the distortion may not cause a linear effect overthe vector path separating the detectors, the approximation maynonetheless help to improve the position estimate. Other such correctionfactors may be used. For example, scaling or correction factors may becollected over a period of time or a number of compressions andaggregated (e.g., averaged together) to correct any measurementdistortions.

Another example approach to distortion compensation using the compounddetector may be to map out a new coordinate system along the path ofmotion. As long as the movement of the detector occurs substantiallyalong or parallel to the path joining the detectors and the separationdistance between the detectors is sufficiently small, one of the twodetectors will move through the initial position of the other. When theposition of the first detector moves into the position previouslyoccupied by the second detector, the measured coordinates of the firstdetector should be equal to or very similar to the coordinates of thesecond detector when it was in the same position. Even if a metaldistorter is present in the environment, the coordinates of the firstdetector will be distorted in the same way the coordinates of the seconddetector were distorted in that same position. Therefore, the seconddetector may map out a new distorted coordinate frame for the firstdetector along the path of travel as shown in FIG. 12.

For example, if the position of the chest is being measured during theadministration of CPR, the compound detector may be positioned on thechest of the patient such that the vector joining the centers of thedetectors is substantially perpendicular to the surface of the chest andsubstantially aligned or parallel with the direction of motion (e.g.,substantially straight down toward the spine). The separation distancebetween the detectors may be small compared to the total distancetraveled. In the case of CPR, an average chest compression may be fivecentimetres and hence, an appropriate separation distance may be, forexample, 10% of the total compression or five millimetres. A smallerseparation distance may provide an improved position resolution.

At the start of a chest compression, the initial position of the seconddetector is measured and defined 20. As the first detector moves downtoward the second detector during the chest compression, its position ismeasured 21. Once the position of the first detector approximatelymatches the initial position of the second detector 22, it may beassumed that the first detector has traveled the five millimetreseparation distance. At this point, the system may once again measureand define a new initial position of the second detector and the processmay be repeated. Even if a source of distortion is present in theenvironment, the distortion should affect both detectors equally at thesame position in space. Consequently, distortion errors may bemitigated, reduced or eliminated. As the chest compression reaches itsdeepest point 23 and begins to travel upwards again 24, the position ofthe first detector may be used to map the coordinate positions.

Once the coordinates along the path of travel have been mapped, positionmeasurements from only one detector may be necessary. However, it may beuseful for the process of mapping out the coordinates along the path oftravel to be performed regularly or repeatedly (e.g., at fixed timeintervals or at trigger events such as start of a compression), so as toaccount for any new distorters entering the environment that were notaccounted for during the first mapping process.

An example method for measuring chest compression depth is nowdescribed. In particular, this method may be suitable for use with asystem having a compound field detector and field generator as describedabove.

The positions of at least two coils (e.g., the coils of the compounddetector) adapted to move in accordance with the patient's chest (e.g.,placed on the chest) are determined. This may be by way of the processorprocessing received response signals from each coil in response to adetected field from the field generator, as described above. Theresponse signals may represent information (e.g., position information)that may be processed by the processor.

The apparent distance between the coils is estimated. For example, theprocessor may determine the apparent distance between the centers of thecoils, based on the response signals received from each coil.

A correction factor is estimated, based on any differences between theapparent distance and the known and fixed distance between the coils.For example, the processor may have the actual fixed distance betweenthe coils of the compound detector stored in its memory. This actualdistance is compared to the apparent distance calculated and acorrection factor is calculated accordingly.

The chest compression depth is determined based on the determinedpositions and the correction factor. For example, the processor maydetermine the position of the compound detector (e.g., by averaging theposition information from the coils of the compound detector) calculatethe apparent chest compression depth using known methods and apply thecorrection factor to get the actual chest compression depth.

The apparent distance between the coils can be determined based on theposition information from each coil at a given time (e.g., as describedabove) or based on the distance as one coil travels from its own initialposition to the initial position of the other coil (e.g., as describedabove).

The two field detectors or coil assemblies in the compound detectorconfiguration may be positioned in such a way that their centres are notaligned along the vector path of the chest compression. For example, inaddition to being spaced apart in a direction parallel to the expecteddirection of motion, the detectors may also be laterally spaced apart,for example as shown in FIG. 13. This may allow the detectors in thecompound detector to have a lesser spacing in the direction of motion.This may also allow the compound detector to be more compact in size. Inthis case, the centres of the field detectors retain a fixed and knownseparation distance 25 along the vector path of the compression but alsohave a fixed and known lateral separation 26 that must be compensatedfor as shown in FIG. 13. Although position resolution may be improved bydecreasing the separation distance, the lateral separation introduces avector component in the separation between the detectors that does notlie along the path of motion. This added vector may complicate themathematical compensation for distortion in the environment.

The compound detector configuration may also include two detectors orcoil assemblies in which each of the two detectors may share one or morecommon perpendicular coils having as shown in FIG. 14. For example, thefirst detector and second detector may each consist of a Z axis coil 27,but may share the same X axis coil 28 and Y axis coil 29. Computationsmay account for the centers of the Z axis coils and the X and Y axiscoils not coinciding. Despite the increased mathematical complexity,this configuration allows the use of fewer coils, which may decreasecosts and/or complexity in manufacturing. For example, in a system thatmay have required six coils 30 as shown in FIG. 15, four may nowsuffice.

A compound detector configuration may allow the system to have a lessersensitivity to the absolute tolerance of the individual detectorassemblies. Instead, the relative tolerance of the two detectors may bethe more important parameter. For example, if the second detector of thecompound detector is mapping the coordinates along the path that will betraveled by the first detector of the compound detector, the moresimilar the two detectors are, the more similar their positioncoordinates will be when they are located at the same position in space.Therefore, the two coil assemblies in the compound detector may be woundso that their number of turns, inductance, resistance, area and otherparameters are relatively closely matched.

Electromagnetic systems inherently suffer from hemisphere or quadrantambiguity. Depending on the number of coils in each detector andgenerator configuration, the received signals may be identical inopposite quadrants or opposite hemispheres. Certain quadrant ambiguitiesmay be resolved by determining the phase of the detected signals.However, when only two detector or generator coils are used instead ofthree, it may be impossible to determine the quadrant of operation. Whenthree detector and three generator coils are used, it may be possible todetermine the quadrant of operation, but not the hemisphere. Using acompound detector configuration may enable certain hemisphereambiguities to be resolved.

For example, in the case of CPR, a change in quadrant as the detector ismoved through the chest compression may cause an unexpected change inposition. If the compression occurs along the Z axis of the detectorcoil, and the generator is positioned such that the detector may movefrom above the generator to down below the generator, an axis 31 may becrossed and a new quadrant 32 may be entered as shown in FIG. 16. Insuch a case, the first detector of the compound detector will cross theaxis before the second detector of the compound detector. Once the firstdetector crosses the axis, its Z coordinate value may begin to increasewhile the Z coordinate value of the second detector continues todecrease. This difference in the direction of travel of each of thedetectors may indicate that the compound detector is crossing an axisand the appropriate signs may be attributed to the measured coordinates.

Providing a compound detector having two detectors with their centresaligned along the path of the chest compression in a known distanceapart may result in a further advantage. The two detectors mayfacilitate the relatively accurate calculation of a plane perpendicularto the movement of the chest compression. Calculating the normal vectorfor the plane may be relatively simple as there are two points along thevector available: the centres of each of the two field detectors in thecompound detector. The normal vector formed by each of the fielddetector centers may be used to relatively efficiently and accuratelycalculate a plane representing the start of the compression. This planemay be used to calculate compression depth as previously described.

In general, although the compound detector is described above asincluding two spaced apart detectors or coil assemblies, it should beunderstood that the compound detector may include more than two spacedapart detectors or coil assemblies. Further, although the detectors orcoil assemblies within the compound detector are shown to be relativelysimilar, they may also be dissimilar in dimension, number of turns,inductance, resistance, etc.

In another embodiment, a sensor 33 or material capable of measuringforce, pressure or contact is provided with the field sensor, forexample between each of the field generator detectors in a two-detectorcompound detector system as shown in FIG. 17. The two field detectorsmay be used to compensate for distortion and improve accuracy of thesystem as described above. The force, pressure or contact sensor is usedto measure the force or contact exerted by the user on the patient'schest during the compression. In the case of a force and/or pressuresensor, the force signal may be used to further improve the measuredposition data. For example, a force or pressure signal may be correlatedwith the position signal to filter out noisy data and signal distortion.

The force, pressure and/or contact sensor may also be used to achieve amore accurate measure of chest recoil. The sensor may be used to detectwhen complete release of the chest following a compression has beenachieved. The field data alone may be used to measure adequate chestrecoil by measuring the extent to which a chest compression returns toits home or starting position. However, the chest may lose complianceover time and the starting position may change over time. In thisinstance, a pressure sensor may help enable more accurate chest recoildetermination.

The position data from the field detectors may also be used to measurethe compliance of the patient's chest. By measuring the total forceapplied by the user to compress the patient's chest a certain distance(as measured by the field detector/generator), a compliance constant maybe determined that correlates the force signal to the chest compressiondepth for that specific patient. This compliance constant may have anumber of uses. For example, if the system determines that its powerlevel is below a given threshold (e.g., where the system is powered by abattery), the system may enter a mode in which position and/or depthinformation is based solely on the force data. For example, the firstcompression of each cycle of thirty may be used to calculate acompliance constant that is used to convert all of the following forcemeasurements into position data.

Beyond power conservation benefits, this design strategy may also allowthe system to operate in a very noisy or highly distorted environmentwith relatively little or no ill-effects. Furthermore, the force sensormay allow the system to continue operation even if the reference sensoris moved during the administration of the chest compressions. Forexample, if the reference sensor is kicked accidentally while CPR isbeing performed, the detected sudden movement may trigger the system toautomatically switch to determining position information using data fromthe force sensor until the reference sensor is determined to berelatively stationary again. This may prevent an interruption in thedetermination of compression depth and/or delivery of depth feedback tothe user. Such sudden movements of the base may be determined bycomparing the force data from the force sensor and position data fromthe position sensors. Any severe incongruities in the two sets of datamay denote a sudden shift in position of the reference sensor.Incongruities between the force sensor and position sensor may alsoindicate that other sources of noise or distortion have entered theoperating environment.

Another example benefit to the incorporation of a force sensor within oron the position sensor is hemisphere ambiguity resolution. Typically,using dot products calculated from the position vectors in the 3×3signal matrix, the quadrant and hemisphere of operation may be resolved.However, the ambiguity resolution using dot products does not eliminateambiguity across hemisphere. Nevertheless, a force or pressure sensormay be used to detect the crossing of a hemisphere boundary of thesystem. When crossing a hemisphere boundary, the position coordinatesmay have the incorrect sign. This may result in an erroneous compressiondepth. It may appear as though the compression is traveling upwardrather than downward. By monitoring the direction of travel with theforce sensor, the signs of the coordinates in each hemisphere may becorrected and the direction of travel may be determined.

Although the previous description incorporating a force, pressure orcontact sensor into the field detector assembly refers to an exampleembodiment in which the two field detectors of a compound detector aresandwiching the force sensor, other embodiments are possible. Forexample, there may only be one field detector rather than a compounddetector, with the force, pressure or contact sensor placed on eitherthe front surface of the field detector (e.g., against the palm of theuser) or the back surface of the field detector (e.g., against thepatient's chest).

A major source of position error in an electromagnetic tracking systemis distortion resulting from the presence of metallic objects in thetracking environment. Specifically, highly conductive metals areparticularly problematic due to the generation of eddy currents withinthem. These eddy currents produce an electromagnetic field that bucksthe magnetic field radiating from the field generator. The use of twofield detectors in a fixed location relative to each other, in acompound detector, may reduce the effect of the distortion as describedabove. However, other methods of distortion compensation may be used inenvironments known to be filled with or encapsulated by large quantitiesof metal. For example, CPR is often performed inside an ambulance. Theframe of the ambulance may consist of aluminum sheets capable ofproducing large eddy currents that may be detected by the field detectorresulting in distorted data. In a known and fixed environment, such asinside an ambulance, the field distortions may be mapped for futurereference by the system.

For example, at each point in the operating environment, the fielddistortion may be measured empirically and a look-up table orcompensation equation (e.g., a polynomial fit) that represents themeasured distortion may be stored within the system's processor memoryas a distortion map. When a certain distortion is measured and matches aknown distortion stored in memory, the processor use the correspondingdistortion map to correct for that known distortion. For example, thecorrect position for that data may be found within the look-up table orby using the distortion compensating equation. The use of the distortionmap may be initiated automatically by the processor (e.g., in responseto the detection of the known distortion) or may be in response toselection by the user. Where a known distortion is detected by theprocessor, the processor may ask the user to confirm whether or not tocorrect for the known distortion (e.g., through a dialog box providedvia the feedback component) before using the distortion map.

For example, since most ambulances have a similar structure and metalliccomposition, it may be possible to incorporate a generic “ambulancemode” into the system that may be activated when performing CPR insidean ambulance, in which the system consults the stored distortion map tocorrect for any distortion arising from the known environment. It mayalso be possible to automatically detect the presence of distortioncaused by the frame of an ambulance. The system may recognize thedistortion signature caused by the typical structure and properties ofan ambulance's shell. Upon detection of the distortion signature, thesystem may automatically begin operation in a distortion-compensationmode by using the pre-determined distortion map corresponding to thedetected distortion signature.

The system may also have an automatic calibration mode where the systemis placed in a specified location (e.g., defibrillator brackets 34 in anambulance) and the environment is surveyed for distortion, for exampleas shown in FIG. 18. This type of distortion compensation may not be aseffective as mapping out the environment in advance, but it does notrequire previously stored information.

A designated distortion mode may also take advantage of a force orpressure sensor embedded within the field detector. For example, in sucha mode, the system may rely more heavily on the force sensor to assistin removing the distortion from the signal. If the patient is moved froma non-distorted location (e.g., pavement) to a distorted environment(e.g., ambulance), the system may use force sensor calibration constantscalculated in the non-distorted environment for proper operation in thedistorted environment.

Metallic objects are one source of potential error and distortion forthe system. Another source of error involves out-of-phase movementbetween the field detector and field generator. Any movement of thereference sensor during the administration of CPR is a potential sourceof error as all measurements are relative to the reference sensor. Thesystem may be configured to recognize any movements of the referencesensor. These movements are typically large and sudden and may be easilyfiltered using various signal processing techniques. Movements of thereference sensor may also be determined by placing a motion detectingsensor, such as an accelerometer, next to or within the referencesensor. The accelerometer may be used to alert the processor that thereference sensor (e.g., field generator) is not stationary. When thereference sensor is moved, the system may temporarily stop sendingposition and/or depth feedback to the user. Once the movement of thereference sensor has ceased, the system may recalibrate a startingposition for calculating compression depth and resume determination ofposition information and resume providing feedback. If a force orpressure sensor is embedded within the position sensor, calibration maybe performed by determining when the chest has been fully released (i.e.when the force exerted on the chest is at a minimum). At this point, thesystem may determine that the compression is at its starting point.Furthermore, a force or pressure sensor may allow the system to continueoperating during a movement of the reference sensor. For example, if thesystem detects that the reference sensor is moved, the position data maybe temporarily based on the force data rather than the data from thefield detector/generator. As previously described, the force sensor maybe first calibrated by using the position data collected from the fielddetector/generator.

When an accelerometer is placed in proximity to the reference sensor,the movement of the reference sensor may be determined and may befiltered out of the position data. For example, a tri-axialaccelerometer may determine motion of the reference sensor in the x, yand z axes and this motion may be subtracted from the motion sensed inthe three axes by the position sensor.

Oftentimes, CPR is performed on a patient supported by a non-rigidsurface such as a mattress. When a chest compression is delivered to apatient on a mattress or other flexible material, the chest undergoestwo distinct movements. The first movement is that of the chest beingcompressed inward by the hands of the user. The second movement is thatof the torso moving into the soft surface of the non-rigid surface. Onlythe movement of the compression itself is useful in forcing bloodthrough the circulatory system of the patient. However, a typical CPRassist system may not be able to distinguish between the two distinctmovements and may measure a larger compression depth than what may haveactually been delivered. Therefore, the system may indicate to the userthat each chest compression is deeper than it is resulting in shallowercompressions.

A number of potential methods may be used for dealing with thissituation. For example, given the three components of a first detector,a second detector and a generator, one component may be adapted to movein accordance with the non-rigid surface (e.g., placed on the mattressbelow the patient), a second component may be adapted to move inaccordance with the patient's chest (e.g., placed on the patient'schest), and the third component may be adapted to be stationary relativeto the patient. The actual depth of compressions may then be determinedby determining the relative motion between the component (e.g., thefirst detector) moving with the non-rigid surface and the component(e.g., the second detector) moving with the patient's chest, for exampleby subtracting the position of one from the position of the other.

In an example embodiment, the field generator may be detachable from thebase unit of the system as shown in FIG. 19 and FIG. 20. For example, ifthe base unit is a defibrillator, the field generator may be removablefrom the defibrillator. The field generator may be positioned on themattress 35 underneath the patient as shown in FIG. 21, adhered to theback of the patient or sandwiched between the mattress and the patient'sback. The field detector 2 may be positioned on the sternum of thepatient. As a chest compression is administered, the field generator maymove with the mattress, while the field detector may move with thecombined motion of the mattress and the chest of the patient. Theposition of the field detector is measured with respect to the fieldgenerator and thus, the movement of the mattress is effectivelyeliminated since the field generator and field detector are bothsubjected to that same movement.

In another example embodiment, there may be two field detectors and onefield generator as shown in FIG. 22. The field generator may be in thebase and one of the field detectors may be placed on the chest of thepatient. The second field detector 36 may be placed on the mattressunderneath the patient, adhered to the back of the patient or sandwichedbetween the mattress and the patient's back. Therefore, the second fielddetector may move with the mattress and may determine the amount ofmotion experienced by the mattress during a chest compression. The firstfield detector 37 may experience the combined motion of the chestcompression and the mattress. Therefore, the processor may subtract themovement of the second field detector from the movement of the firstfield detector thereby eliminating the movement of the mattress from thechest compression depth measurement.

In another example embodiment, there may not be a base unit. The systemmay comprise a field generator and a field detector in which the fieldgenerator is placed underneath the patient and the field detector isplaced on the chest of the patient as shown in FIG. 23. As described inthe previous embodiments, the movement of the mattress may be easilysubtracted from the compression depth calculation. Other embodiments inwhich the placement of the field generator and field detector areinterchanged are also possible.

Methods of compensation may be required for other possible situations.For example, the chest of the patient may lose compliance over time. AsCPR is performed, the chest may sink and there may be less elasticity inthe internal structures. This may result in a drift of the actualstarting position of the chest compressions over time. The system maycompensate for a loss of chest compliance and a change in startingposition of the compressions by recalibrating the start position (e.g.,top of the chest compression) before each cycle of chest compressions.For example, if each CPR cycle consists of thirty compressions and tworescue breaths, the system may recalibrate the compression startposition during the administration of the two breaths. If continuouscompressions are being delivered, absent of any interruptions, thesystem may calibrate using a force or pressure sensor. The system maydetermine the starting position of a compression by detecting the pointwhere the minimum amount of force is exerted by the user on the chest.

Oftentimes CPR must be performed in a moving environment. For example,CPR is regularly administered inside a moving ambulance or medicalhelicopter. Furthermore, CPR may be performed inside larger vehiclessuch as trains, planes or large ships. Current compression depth methodsemploying accelerometers may register the external movements of thesevehicles as a part of the chest compressions. The accelerometer measuresacceleration relative to the Earth and it may be relatively difficultfor the accelerometer to isolate the compression movement from that ofthe vehicle. By using an external reference sensor, the movement due toa vehicle may be easily eliminated. For example, the reference sensormay be placed within the vehicle or moving environment and allmeasurements by the position sensors are made relative to the referencesensor. Therefore, any movement experienced by both the reference sensorand the position sensors may be effectively ignored or taken intoaccount by the system.

Other moving environments beyond vehicular transport of a patient arepossible. For example, a patient may be transported on a gurney,stretcher or backboard 38. As shown in FIG. 24, the reference sensor maybe placed on or within the backboard 38. The position sensor may beconnected with a connector 39 into the backboard and the feedback 40 maybe provided on the pad housing the position sensor.

The orientation of the position sensor may be determined usingcalculations disclosed in U.S. Pat. No. 4,314,251, the disclosure ofwhich is hereby incorporated in its entirety by reference. The roll 41,pitch 42 and yaw 43 may be used to determine the three dimensionalconfiguration of the position sensor as shown in FIG. 25. During CPR,the starting configuration and orientation of the position sensorrelative to the chest of the patient may be known. When placed inside apuck or pad 6 positioned on the patient's chest, axes 15 or othermarkings may be labeled on the position sensor housing indicating properorientation of the system. The tilt of the position sensor may then becalculated and factored into the depth calculation. If the positionsensor is placed unevenly on the chest, the orientation angles may beused to correct the depth calculation. Furthermore, the calculatedangles may be used to rotate the frame of reference for the positionsensor. For example, the position information may be rotated into theposition sensor's reference frame to simplify calculations and improveaccuracy.

CPR is physically demanding and user fatigue often results shortly aftercommencing chest compressions. Ineffective technique and improperphysical form may lead to a faster onset of fatigue and pain associatedwith prolonged administration of CPR. When delivering compressions, theuser should have his or her shoulders 44 positioned directly over thebody of the patient with his or her arms 45 locked, straight andperpendicular to the patient's chest as shown in FIG. 26. The system maybe used to monitor the angle 46 of chest compressions by housing theposition sensor within a wearable embodiment of the system as shown inFIG. 27. For example, the position sensor may be housed in a wrist bandor glove that positions the sensor on the wrist of the user. The roll,pitch and yaw angles may be used to determine the relative orientationof the user's arm. The user may then be prompted to adjust his or herarm angle 46 to maximize the transfer of force during CPR and reduceuser fatigue. To reduce distortion and error in the angle measurement,the wearable system may incorporate a bend sensor 47 that varies itsresistance with the degree of bend. The sensor may be used to measurethe bend of the wrist of the user. This data may be correlated with thefield data to improve accuracy. The bend sensor serves a similarfunction for angle as the force or pressure sensor does for depth.

Different methods of combining various sensors with the field detectorand field generator may be used to improve positional accuracy andcorrect for distortion and/or noise. The system may also be combinedwith other sensors. As previously described, accelerometers may be usedto detect external motion such as movement of the base unit or referencesensor when it is moved during CPR. An accelerometer may also be used toverify the data collected from the field detectors and eliminatemetallic sources of distortion. The system may also be combined withpulse oximetry to monitor blood flow through the patient and correlatethe blood flow to the field data for increased accuracy. For example,the reference sensor may be incorporated into a pulse oximetry unit 48on the patient's finger or forehead as shown in FIG. 28.

The system may also incorporate chest impedance measurements. Chestimpedance 49 may be used to detect motion of the patient's chest bymeasuring impedance between two electrodes, such as defibrillator pads50 as shown in FIG. 29. Chest impedance measurements may serve a similarfunction as the force sensor measurements in the present system. Thechest impedance measurements may be correlated to the field data toimprove accuracy and remove sources of distortion from the measurements.Furthermore, the field data may be used to calculate calibrationconstants for the chest impedance measurements that correlate the chestimpedance to compression depth. In this way, the chest impedance may beused for depth data when the reference sensor is moved during theadministration of chest compressions or if the system is used in ahighly distorted environment. As many defibrillators already have chestimpedance measurements incorporated within them, the field data may beeasily adapted to transform the impedance data into a more useful andmore accurate parameter.

The presently disclosed system may be adaptable for different patients.As the position sensor placed on the patient's chest may be made verysmall and lightweight, it may be adapted for used on an infant or adult.For example, the position sensor may be placed in a removable housing 51for adult CPR as shown in FIG. 30. The housing may provide a largersurface area and be configured for two handed CPR. When removed, theposition sensor may be smaller and more suitable for two-finger CPR onan infant.

The presently disclosed system may be implemented in a number ofembodiments and feedback may be transmitted to the user in variousforms, via a feedback component (e.g., screen, speaker, lights, buzzer,etc.). For example, the feedback may be audible, visual or both. Thefeedback may be displayed on an LCD display within the base unit. It isalso possible that the feedback be incorporated into the position sensorpad 6 itself as shown in FIG. 31. The feedback on the pad may be adisplay or may be in the form of LED graphics 52. The audio feedback maybe delivered through a speaker within the base or within the positionsensor pad. The audio feedback may be in the form of voice promptsand/or a pacing metronome.

Beyond CPR related prompting, other information may be relayed to theuser. For example, the system may be capable of detecting that thedistance between the position sensor and the reference sensor is outsidethe operating range. In this scenario, the system may prompt 53 the userto move the base containing the reference sensor closer to the positionsensor as shown in FIG. 32.

The embodiments of the present disclosure described above are intendedto be examples only. Alterations, modifications and variations to thedisclosure may be made without departing from the intended scope of thepresent disclosure. In particular, selected features from one or more ofthe above-described embodiments may be combined to create alternativeembodiments not explicitly described. All values and sub-ranges withindisclosed ranges are also disclosed. The subject matter described hereinintends to cover and embrace all suitable changes in technology. Allreferences mentioned are hereby incorporated by reference in theirentirety.

1. A system for determining a depth of compression of a chest of apatient receiving chest compressions, the system comprising: a fieldgenerator adapted to generate a field; a compound field detectorincluding at least two coils at a fixed distance from each other, thefield detector adapted to generate a response signal indicative of anyone of the at least two coils detecting the field; and a processoradapted to determine from the response signal position information forthe field detector relative to the field generator, and to determine thechest compression depth from the determined position information;wherein one of the field generator and the field detector is a positionsensor adapted to move in accordance the chest as the chest is receivingthe compressions, and the other of the field generator and the fielddetector is a reference sensor adapted to be stationary relative to thepatient.
 2. The system of claim 1, wherein each coil in the compoundfield detector comprises windings, and the windings of each coil aresubstantially parallel to the windings of each other coil.
 3. The systemof claim 1, wherein each coil in the compound field detector compriseswindings, each coil having a longitudinal axis perpendicular to a planedefined by the respective windings, wherein the longitudinal axis ofeach coil is substantially parallel with a direction of the chestcompressions.
 4. The system of claim 1, wherein the field generator iscoupled with the field detector via a wire.
 5. The system of claim 1,wherein the field generator is coupled wirelessly with the fielddetector.
 6. The system of claim 1 wherein the processor is furtheradapted to receive signals from each of the coils, determine an apparentdistance between the coils, and determine a correction factor to correctfor distortion in the position information, the correction factor beingdetermined based on any differences between the apparent distance andthe fixed distance between the coils.
 7. The system of claim 6 whereinthe processor is further adapted to define initial positions for each ofthe coils at a given time, wherein the apparent distance is determinedbased on apparent distance traveled by one of the at least two coilsfrom its own initial position to an initial position of another of theat least two coils.
 8. The system of claim 1 wherein the chestcompression depth is calculated by forming a plane in spacesubstantially perpendicular to a direction of the chest compressiondepth.
 9. The system of claim 1 wherein the field detector is theposition sensor and the field generator is the reference sensor.
 10. Thesystem of claim 1 wherein coordinates of the position information arerotated into a frame-of-reference of the position sensor.
 11. The systemof claim 10 wherein the chest compression depth is calculated bydetermining distance from a starting position coordinate of chestcompression to a final position coordinate of chest compression, thefinal position coordinate corresponding to a final depth of chestcompression.
 12. The system of claim 1 wherein the field generator isadapted to be placed on the chest of the patient and the field detectoris adapted to be stationary relative to the patient.
 13. The system ofclaim 12 wherein the position information is in a frame-of-reference ofthe field generator and the chest compression depth is calculated bydetermining the distance from a starting position coordinate of chestcompression to a final position coordinate of chest compression, thefinal position coordinate corresponding to a final depth of chestcompression.
 14. The system of claim 1 wherein the position informationincludes measured orientation angles of the position sensor.
 15. Thesystem of claim 14 wherein an angle of a direction of chest compressionrelative to an axis perpendicular to the patient's chest is calculatedusing the orientation angles.
 16. The system of claim 1 wherein theposition sensor is wearable.
 17. The system of claim 16 wherein a bendsensor is provided with the position sensor for sensing an orientationof the position sensor.
 18. The system of claim 17 wherein measured datafrom the bend sensor is used to correct for noise and distortion fromorientation information measured by the position sensor.
 19. The systemof claim 1 wherein a distortion map of known distortions in an operatingenvironment is stored in the processor, and the processor is furtheradapted to correct for the known distortions using the distortion map.20. The system of claim 19 wherein the processor is further adapted todetect the presence of known distortions in the environment and, inresponse to the detected presence, to automatically initiate use of thedistortion map to correct for the known distortions.
 21. The system ofclaim 19 wherein the distortion map is created by placing the fieldgenerator and field detector in the operating environment anddetermining electromagnetic fields at specific locations within theenvironment.
 22. The system of claim 19 wherein the operatingenvironment is inside an ambulance.
 23. The system of claim 1 furthercomprising a second field detector for taking into account any motion ordisplacement of a non-rigid surface supporting the patient.
 24. Thesystem of claim 23 wherein a first one of the compound field detector,the second field detector and the field generator is adapted to move inaccordance with the non-rigid surface, a second one of the compoundfield detector, the second field detector and the field generator isadapted to move in accordance with the chest of the patient, and a thirdone of the compound field detector, the second field detector and thefield generator is adapted to be stationary relative to the patient. 25.The system of claim 24 wherein motion or displacement of the non-rigidsurface is taken into account by determining the relative motion betweenthe first one of the compound field detector, the second field detectorand the field generator adapted to move in accordance with the non-rigidsurface, and the second one of the compound field detector, the secondfield detector and the field generator adapted to move in accordancewith the chest of the patient.
 26. The system of claim 1 furthercomprising at least one of a force sensor and a pressure sensor providedwith the position sensor.
 27. The system of claim 26 wherein theprocessor is further adapted to correct for at least one of noise,distortion and hemisphere ambiguity in the position information usingdata from the at least one of the force sensor and the pressure sensor.28. The system of claim 26 wherein the processor is further adapted tocalculate a calibration constant for data from at least one of the forcesensor and the pressure sensor using the position information.
 29. Thesystem of claim 28 wherein the calibration constant transforms the datafrom the at least one of the force sensor and pressure sensor into atleast one of position data and depth data.
 30. The system of claim 29wherein the processor is further adapted to calculate the chestcompression depth using the data from the at least one of the forcesensor and the pressure sensor in the absence of position information.31. The system of claim 30 wherein the system receives power from apower source, and the processor is further adapted to turn off power tothe field generator and use the data from the at least one of the forcesensor and the pressure sensor to conserve system power.
 32. The systemof claim 26 wherein the processor is further adapted to determine chestrecoil using data from the force sensor.
 33. The system of claim 1further comprising an accelerometer provided with the reference sensor.34. The system of claim 33 wherein the accelerometer is adapted todetect movement of the reference sensor.
 35. The system of claim 34wherein movement detected by the accelerometer is used to correct forany movement of the reference sensor.
 36. The system of claim 1 whereinthe processor is further adapted to receive chest impedance data and touse the chest impedance data to correct for any noise and distortion inthe position information.
 37. The system of claim 1 further comprising ahousing within which the position sensor is removably contained.
 38. Thesystem of claim 37 wherein the removable housing is removable forperforming CPR on an infant.
 39. The system of claim 1 wherein thesystem comprises a feedback component for providing feedback to a CPRadministrator based on at least one of the determined positioninformation and the determined chest compression depth.
 40. The systemof claim 39 wherein the feedback comprises at least one of visual,audio, and tactile prompts.
 41. The system of claim 1 wherein thereference sensor and the processor are provided in a base unit, and theposition sensor is in communication with the base unit.
 42. The systemof claim 41 wherein a power source is provided in the base unit, and anauxiliary power source is provided with the position sensor.
 43. Thesystem of claim 41 wherein the base unit comprises a feedback componentfor providing feedback to a CPR administrator based on at least one ofthe determined position information and the determined chest compressiondepth.
 44. The system of claim 43 wherein the feedback comprises atleast one of visual, audio, and tactile prompts.
 45. A method fordetermining a depth of compression of a chest of a patient receivingchest compressions, the method comprising: determining the positions ofat least two coils adapted to move in accordance with the chest;estimating an apparent distance between the coils; estimating acorrection factor based on any differences between the apparent distanceand a known distance between the coils; and determining the chestcompression depth based on the determined positions and the correctionfactor.
 46. The method of claim 45 wherein the coils are provided in acompound field detector used in a system for determining chestcompression depths, the system further comprising a field generator anda processor, wherein the coils are adapted to generate response signalsin response to a field generated by the field generator, and wherein theprocessor is adapted to receive signals from the coils and to carry outthe method using information represented by the received signals. 47.The method of claim 45 wherein the known distance is substantiallyparallel to a direction of chest compression.
 48. The method of claim 45wherein initial positions are defined for each coil at a given time, anddetermining the apparent distance is based a determination of apparentdistance traveled by one of the at least two coils from its own initialposition to an initial position of another one of the at least twocoils.
 49. A compound field detector for determining a depth ofcompression of a chest of a patient receiving chest compressions, thedetector comprising: at least two coils at a fixed distance from eachother; wherein the detector is adapted to generate a response signalindicative of any one of the at least two coils detecting a field. 50.The detector of claim 49, wherein each coil comprises windings, and thewindings of each coil are substantially parallel to the windings of eachother coil.
 51. The detector of claim 49, wherein each coil compriseswindings, each coil having a longitudinal axis perpendicular to a planedefined by the respective windings, wherein the longitudinal axis ofeach coil is substantially parallel with a direction of the chestcompressions.
 52. The detector of claim 49, further comprising a wirefor coupling to a field generator.
 53. The detector of claim 49, whereinthe detector is further adapted to couple wirelessly to a fieldgenerator.
 54. The detector of claim 50 further comprising at least oneperpendicular field detecting coil having windings perpendicular to andsurrounding the at least two parallel coils.
 55. The detector of claim50 wherein each parallel coil is surrounded by up to two perpendicularfield detecting coils having windings perpendicular to the parallelcoils and perpendicular to each other perpendicular coil.
 56. Thedetector of claim 54 wherein the parallel coils share at least onecommon perpendicular coil.